This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-202549, filed Jul. 3, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an ultrasound diagnostic apparatus for transmitting/receiving ultrasound waves by using a pulse compression technique.
2. Description of the Related Art
A pulse compression technique is a technique developed in the field of radars. An increase in maximum radar range requires an increase in transmission pulse length. An increase in maximum radar range, however, degrades distance resolution. Pulse compression has been developed to attain an increase in maximum radar range and an improvement in distance resolution. Pulse compression is performed by using a transmission pulse having a long pulse length obtained by performing special modulation inside a pulse. The pulse length is substantially decreased by demodulating the reception signal.
Pulse compression schemes are classified into a linear frequency modulation pulse compression scheme and a phase-coded pulse compression scheme. In the linear frequency modulation pulse compression scheme, a chirp signal that is frequency-modulated such that the frequency linearly changes is transmitted. The reception signal is demodulated by a circuit having a frequency/delay time characteristic reverse to transmission frequency modulation. With this operation, dispersed frequency components are concentrated to one point.
In the phase-coded pulse compression scheme, the phase of a reference waveform signal is discretely modulated (0, xcfx80) in accordance with a code series (a series of 1 and xe2x88x921). The phase of the reception signal is modulated with a code series reverse to the transmission code series.
As is known, the waveform after pulse compression does not theoretically have a single component, and small components called range sidelobes appear on both sides of a central component. As a means for reducing such range sidelobes, a pair of code series called a Golay codes has been found.
The Golay codes are constituted by a pair of complementary code series (FIGS. 1A and 1B). A reference signal 100 phase-modulated in accordance with one code (FIG. 1A) is transmitted, and a reception signal 101 (FIG. 1C) is acquired. Likewise, at the next rate, a signal 200 phase-modulated in accordance with the other code (FIG. 1B) is transmitted, and a reception signal 201 (FIG. 1D) is acquired. The reception signal 101 is convoluted with the corresponding signal 100 to generate a demodulated signal 102 (FIG. 1E). Likewise, the reception signal 201 is convoluted with the corresponding phase-modulation signal 200 to generate a demodulated signal 202 (FIG. 1F). The reception signals 102 and 202 are added (FIG. 1G). With this operation, a signal 300 in which range sidelobes cancel out each other can be obtained.
Studies have been made to apply the above pulse compression technique, especially the phase-coded pulse compression technique using a Golay code, to ultrasound diagnosis.
This application is, however, hindered by causes unique to ultrasound diagnosis. The greatest cause is the motion of the tissue (reflecting/scattering body). The motion of the tissue between two rates causes a phase difference corresponding to the motion between signals with the two rates. As a consequence, range sidelobes remain.
In order to solve this problem, a phase change due to the motion of the tissue between the rates must be obtained, and phase compensation must be performed with respect to a pair of reception signals. As a typical method for such operation, a method using a Doppler technique is available, in which transmission/reception is repeated at least at two rates, the complex number of a signal with one rate at each depth is multiplied by the complex number of a signal with the other rate at the corresponding depth, and a phase argument is obtained from the multiplication result. In the autocorrelation method, similar processing is performed between a plurality of rates to obtain a complex vector product. This case can be regarded as a special case to which the autocorrelation method is applied, in which the number of data is two. When the obtained phase argument is normalized with 2xcfx80, and the product of the normalized value and the wavelength of a barycentric frequency representing the fundamental wave is calculated, the displacement of the tissue between the two rates can be obtained.
This phase compensation (motion compensation) technique cannot be applied to the phase-coded pulse compression scheme using a Golay code. Since different transmission waveforms are used, reception signals differ in their waveforms between the rates even if the scattering body remains the same. This makes it impossible to extract only a phase difference due to the motion of the scattering body at each portion between signals.
It is an object of the present invention to realize the use of a phase-coded pulse compression scheme for ultrasound diagnosis.
According to the first aspect of the present invention, there is provided an ultrasonic diagnostic apparatus comprising a transmitting/receiving unit configured to transmit an ultrasound wave to an object to be examined at a first rate in accordance with a first code, receive a first reception signal from the object, transmit an ultrasound wave to the object at a second rate in accordance with a second code complementary to the first code, and receive a second reception signal from the object, a first processor configured to convolute the second code in the first reception signal, convolute the first code in the second reception signal, and detect a phase difference between the two signals, the phase difference representing a motion of a tissue of the object between the first and second rates, a second processor configured to compensate at least one of the first and second reception signals on the basis of the phase difference, a third processor configured to convolute the first and second codes in the compensated first and second reception signals, respectively, add the two signals, and generate a third reception signal, and a unit configured to generate image data on the basis of the third reception signal.
According to the second aspect of the present invention, there is provided an ultrasonic diagnostic apparatus comprising a transmitting/receiving unit configured to transmit an ultrasound wave to an object to be examined at a first rate in accordance with a first code, receive a first reception signal from the object, transmit an ultrasound wave to the object at a second rate in accordance with a second code complementary to the first code, and receive a second reception signal from the object, a first processor configured to convolute the second code in the first reception signal, convolute the first code in the second reception signal, and detect a phase difference between the two signals, the phase difference representing a motion of a tissue of the object between the first and second rates, a second processor configured to convolute the first and second code in the compensated first and second reception signals, respectively, add the two signals upon giving a time shift corresponding to the phase difference, and generate a third reception signal, and a unit configured to generate image data on the basis of the third reception signal.
According to the third aspect of the present invention, there is provided an ultrasonic diagnostic apparatus comprising a transmitting/receiving unit configured to transmit an ultrasound wave to an object to be examined at a first rate in accordance with a first code, receive a first reception signal from the object, transmit an ultrasound wave to the object at a second rate in accordance with a second code complementary to the first code, and receive a second reception signal from the object, a first processor configured to convolute the second code in the first reception signal, convolute the first code in the second reception signal, and detect a cross-correlation function between the two signals, the cross-correlation function representing a motion of a tissue of the object between the first and second rates, a second processor configured to compensate at least one of the first and second reception signals on the basis of the cross-correlation function, a third processor configured to convolute the first and second code in the compensated first and second reception signals, respectively, add the two signals, and generate a third reception signal, and a unit configured to generate image data on the basis of the third reception signal.
According to the fourth aspect of the present invention, there is provided an ultrasonic diagnostic apparatus comprising a transmitting/receiving unit which transmits an ultrasound wave to an object to be examined in accordance with a first code signal with a first rate, receives a first reception signal, transmits an ultrasound wave to the object in accordance with a second code signal with a second rate, and receives a second reception signal from the object, a first processor which estimates a motion of a tissue in the object between the first and second rates on the basis of the first and second reception signals and the first and second code signals, a second processor which compensates the first and second reception signals on the basis of the estimated motion, and a unit which generates image data on the basis of the compensated first and second reception signals.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.